Fresh doubts emerged over England players' participation in the Indian Premier League after IPL chief Lalit Modi demanded that they play for at least one month in the Twenty20 tournament, according to a report.Modi has reversed the understanding the England and Wales Cricket Board thought it had reached with him under which ECB centrally contracted players would appear in the IPL for no more than 15 days, a report in The Guardian said."The board believed Modi had agreed that being available for 15 days of the seven-week event would be sufficient but he has set the minimum at one month. This would mean half-dozen players thought to be targeting an IPL deal, including Kevin Pietersen, might have to choose between England and IPL," the report said.Professional Cricketers' Association chief executive Sean Morris described the development "a serious concern"."This development is a serious concern. We are working through this with ECB but clearly it is a concern to India and has a potential impact on English cricket. If we are the only country not to have players appearing in the IPL then that would not be a good prospect," Morris said.If the ECB gives its players a month IPL window they would miss the first Test against West Indies starting at Lord's on May 6. The IPL is slated for April 10 to May 29.Pietersen held talks with a representative of Bangalore Royal Challengers when England played their one-day match there last November but at least two other IPL franchises are reportedly interested in signing the former England captain.